When upgrading MySQL on OS X (anno domini 2010), I learned the hard way that the “data” folder containing your databases and tables does not get moved. Look into /usr/local; you will probably have the old mysql-* folder there containing just the “data” folder. /usr/local/mysql is a symlink to the current MySQL installation, so move its data folder elsewhere, and put the old one in there.
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via blog.vucica.net